The long-term objective of the proposed research is to further our understanding of the mechanisms underlying psychophysical masking. In particular, this project seeks to clarify the role of basilar membrane compressive nonlinearity on growth of masking by altering the nonlinearity through the ingestion of aspirin, an ototoxic drug known to adversely affect outer hair cell function, and hence basilar membrane compression. The goals of this research are to (1) gain insight into the role of basilar membane compressive nonlinearity on growth of masking in forward masking for on-frequency (masker and signal equal in frequency) and off-frequency (masker well below the signal in frequency) maskers, (2) gain a better understanding of the differences in the processing underlying growth of masking in simultaneous and forward masking for off-frequency maskers, and (3) evaluate the legitimacy of using temporary aspirin-induced hearing loss as a model for permanent sensorineural hearing loss. Results from the proposed study will resolve issues fundamental to our understanding of the role of cochlear nonlinearity in psychophysical masking. Moreover, the ability to use aspirin-induced hearing loss as a model for sensorineural hearing loss will be a valuable tool for future research on the processing abilities of hearing-impaired individuals. The proposed experiments will examine growth of masking in forward and simultaneous masking in normal-hearing individuals prior to, and during four days of moderate aspirin use. The aspirin will likely induce a temporary, mild-to-moderate cochlear hearing loss, and changes to the growth-of-masking functions with aspirin will provide insight into the influence of basilar membrane nonlinearity on the functions. The results from individuals with temporary aspirin-induced hearing loss will be compared with those from individuals with permanent sensorineural hearing loss of similar degree and configuration. This comparison will test the hypothesis that sensorineural and aspirin-induced hearing losses affect growth of masking similarly, supporting the aspirin-induced hearing loss as a model for sensorineural hearing loss. All testing will be completed using standard psychoacoustic forced-choice adaptive procedures.